In recent years a migration is taking place from simple stand-alone lighting devices towards lighting systems comprised of a plurality of lighting devices equipped with automatic switching and communication facilities, such as time-controlled switching and wireless communication, for example. Such systems of interconnected lighting devices or luminaires offer a wide range of control functions, but are more expensive than basic luminaires with a simple on-off function only.
In situations in which a more basic functionality is sufficient, but still an easy adaptation of the settings of the lighting operation is required, such as a time-controlled lighting operation, the lighting device may be made configurable via a local point-to-point wireless connection, such as a Bluetooth™ or Wi-Fi connection, for example. This adds less cost and still offers the possibility to change the settings of the luminaire electronically, instead of physically or manually adapting the settings with each lighting device.
If the luminaire is equipped with a timing circuit, such as a real-time clock, the device can run lighting schedules that are not only time-dependent, but may also dependent on the day of the week, for meeting different lighting demands on weekdays or weekends, for example, or may be dependent on the day of the year to include public holidays and to compensate for daylight savings in case of public outdoor lighting or safety lighting operation, for example.
Outdoor luminaires for street lighting, for example, may connect to a separately switched main power supply or grid, such as the case in many European countries, by which the lighting device is periodically powered in accordance with a power supply cycle having a power-on time and a power-off time. Accordingly, the lighting device and hence the timing circuit is not continuously powered. To keep the timing circuit running during the power-off periods, an independent backup or auxiliary power source is needed for additionally powering the timing circuit. For this purpose, a chargeable battery or a sufficiently large (super) capacitor may be used, for example, that keeps the timing circuit ‘alive’ in between power supply cycles.
If the backup power is supplied by a non-chargeable battery, a sufficient capacity must be available to span the entire economic lifetime of the luminaire, in order to avoid a regular, costly and labour-intensive replacement of the battery in each luminaire. This capacity must be even larger due to the fact that the battery has self-leakage that over the years can add to become a substantial portion of the overall capacity loss. Moreover tolerances in battery capacity, consumption of the clock chip and temperature cycles will increase the capacity requirement even more. Therefore a relatively large battery must be used, which adds not only to cost but also to the size of the lighting device.
One way to overcome this is to make use of a (re-)chargeable battery that gets charged whenever the lighting device is on mains power or by using a temporary power storage, such as a capacitor having a particular power storage capacity. With the typical power consumption of current timing circuits, such as real-time semiconductor chipsets and available capacitance values, it is possible to keep the timing circuit powered for spanning several days or even some weeks in-between power supply cycles which, in most practical cases, appears sufficient.
It can, however, happen in practice that the power-off period of the main power supply is longer than the power life-time of a battery or charge storage capacitor, for example in case of a serious power failure or power outage that is not repaired within this period. For these cases it is desired that some mechanism is available to synchronise the time function in each lighting device.
US patent application US2016/0021721 discloses a street lighting device having a processor controlled self-calibrating clock device and stored historical ambient light data. The street light senses ambient light conditions and matches same to the historical data for determining a current date, day and month of a year.
Sensors for measuring ambient light conditions, for example, besides adding to the component costs of a lighting device, also put requirements on the design of the housing of the lighting device, such that sufficient light can impinge on a sensor, and for avoiding accumulation of dirt and moisture by which the entrance of light could be decreased over time.